CH₃⁺ Formation in the Dissociation of Energy-Selected CH₃F⁺ Studied by Double Imaging Electron/Ion Coincidences

Abstract

The dissociation of energy-selected methyl fluoride ion CH3F(+) along the CH3(+) and F formation channel has been investigated in detail in the 12.2-19.8 eV energy range. Three low-lying electronic states of the CH3F(+) ion, X(2)E, A(2)A1, and B(2)E, were prepared and analyzed by the method of double imaging photoelectron photoion coincidence (i(2)PEPICO). Three types of CH3(+) fragment ions corresponding to the dissociation of X(2)E, A(2)A1, and B(2)E electronic states have been observed and identified through their kinetic energy release distribution (KERD) curves, showing that the dissociation of the CH3F(+) ion in the different electronic states along the CH3(+) formation channel is state-specific, even outside of the Franck-Condon region. Highly excited vibrational levels of the X(2)E ground state can be populated through a resonant or near-resonant autoionization process and undergo vibrational predissociation into CH3(+) fragment ions. The A(2)A1 first excited state is repulsive along the F-loss coordinate, and its dissociation is direct and fast. The CH3F(+) ions in the B(2)E excited state first undergo an internal conversion toward the hot cation ground state X(2)E and then statistically dissociate into CH3(+) and F fragments.

Charge transfer in dissociating iodomethane and fluoromethane molecules ionized by intense femtosecond X-ray pulses

“…(1), adapted for CH 3 F. The energies are then converted into time of flight by accounting for the geometry of the spectrometer. The asymptotic energies of the fluorine ions are chosen as ( i ) 0.4 eV (black curve), ( ii ) 0.84 eV (dashed cyan), and ( iii ) 0.05 eV (dashed blue), in accordance with ( i ) the mean kinetic energy of the ${\text{CH}}_3^{+}$ fragments measured in this work, ( ii ) the maximum of the energy distribution reported for single-photon fragmentation at 18.5 eV photon energy 44 (∼four 266 nm photons), and ( iii ) the energy for ion pair production 22 that has a maximum cross section at photon energies around 13–14 eV 45 (∼three 266 nm photons). Negative fluorine can be produced by excitation to the 3A 1 and 4A 1 superexcited states, followed by internal conversion to the 2A 1 state via avoided crossings, resulting in dissociation into the ion pair ${\text{CH}}_3^{+}$ + F − 21 .…”

Adiabatic ionization energies of the overlapped A2A1and B2E electronic states in CH3Cl+/CH3F+measured with double imaging electron/ion coincidence

“…3,4,14 The techniques of threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO) have been frequently employed to measure the values of ionization energy and fragment appearance energy due to the high collection efficiency of threshold electrons associated with a high and constant electron energy resolution. [22][23][24][25][26][27][28][29][30][31][32][33][34] The valence shell molecular configurations of CH 3 Cl and CH 3 F in their X 1 A 1 electronic ground state are known as (1a 1 ) 2 (2a 1 ) 2 (1e) 4 (3a 1 ) 2 (2e) 4 , where their core shells have been neglected for simplicity. Indeed, in photoionization processes the molecular ions may be prepared in different states depending on the photon energy and transition possibility.…”

“…Indeed, in photoionization processes the molecular ions may be prepared in different states depending on the photon energy and transition possibility. The X 2 E ground states of both CH 3 Cl + and CH 3 F + , have vibrational structures visible in their PES 33 and TPES 22,23 and their adiabatic ionization energies (AIEs) are located at 11.289 AE 0.003 and 12.533 AE 0.006 eV, 33 respectively. In particular, for the overlapped states commonly existing in the excited electronic states, 1,4 due to the small energy difference between them, it is still a challenge for both experimental and theoretical scientists to accurately measure or predict their ionization energies.…”

“…In particular, for the overlapped states commonly existing in the excited electronic states, 1,4 due to the small energy difference between them, it is still a challenge for both experimental and theoretical scientists to accurately measure or predict their ionization energies. 22,23 The A 2 A 1 and B 2 E states of CH 3 Cl + partially overlap and those of CH 3 F + totally overlap, due to a small energy difference between them and the natural width of the dissociative states, so that they cannot be separated even with high resolution electron spectroscopy, a situation frequently encountered in other molecular systems. [22][23][24][25][26][27][28][29][30][31][32][33][34] The valence shell molecular configurations of CH 3 Cl and CH 3 F in their X 1 A 1 electronic ground state are known as (1a 1 ) 2 (2a 1 ) 2 (1e) 4 (3a 1 ) 2 (2e) 4 , where their core shells have been neglected for simplicity.…”

“…22,23,32 The appearance energies of CH 3 + fragment ions in the dissociation of CH 3 Cl + and CH 3 F + are at 13.38 and 14.54 eV, respectively, lying in the Franck-Condon gap. 22,23,32 The appearance energies of CH 3 + fragment ions in the dissociation of CH 3 Cl + and CH 3 F + are at 13.38 and 14.54 eV, respectively, lying in the Franck-Condon gap.…”